A conventional image forming apparatus, such as a copying machine, has external devices, such as a DF (Auto Document feeder) and a print engine, so as to execute a function, such as scanning and printing. It is necessary for a copying machine to execute a plurality of data processings at a constant data transfer rate, in accordance with the processing speeds of these external devices. The copying machine has a plurality of processing blocks in order to perform these data processings. The respective processing blocks are connected to a common memory, via a plurality of channels connected to a common bus, so as to construct a flow of data. For example, when one data processing out of the above-described plurality of data processings is executed at a data transfer rate which is extremely lower than the above-described constant data transfer rate, the copying machine generates an abnormal image such as missing of an image. Accordingly, a data transfer apparatus of the copying machine monitors a transfer rate of image data in each of the above-described plurality of channels, and takes such a measure as to ensure the band by raising an access priority of a channel to the common bus, when the data transfer rate of the channel is lower than a threshold value.
In order to ensure the band by raising the access priority of a channel to the common bus as described above, the data transfer control apparatus is configured to change a priority of an arbitrator, by the difference between a rate monitoring result (measurement band) and a target rate (target band). For this configuration, the data transfer control apparatus cannot perform fine adjustment of the transfer rate between channels of a high priority, in some cases. For example, a request of a channel of a low priority is sometimes inputted between non-continuing requests of a channel of a high priority. Further, in a conventional data transfer apparatus, when a usage rate of a bus is high, it is difficult to perform a rate control which is balanced between channels. In other words, in the conventional data transfer apparatus, there is a problem such that it is difficult to perform data transfer finely and in good balance against the variation of the transfer rate.